Patent GB-1494339-A (GB)
Title: Digital multiplexing system
IPC: H04J3/07
Filed: 1974-11-25
Abstract: 1494339 Time division multiplexing systems FRANCE MINISTRY OF POST & TELECOMMUNICATIONS AND SOC ANON DE TELECOMMUNICATIONS 25 Nov 1974 [27 Nov 1973] 51018/74 Heading H4M In a digital multiplexing system, incoming plesiosynchronous low rate component signals comprising only information bits and having originating rates departing from a slightly higher given nominal low rate are time multiplexed bit by bit into intermediate rate signals and a plurality of the intermediate rate signals are time multiplexed bit by bit into a single outgoing high rate signal by stuffing techniques which avoid the need for separately timed storage and stuffing arrangements in the two stages. The low rate component signals are stored and stuffed in dependence on the difference between originating rate and nominal low rate and read out from the stores to a multiplexing arrangement in which they are multiplexed into intermediate rate signals. Filling bits comprising framing bits, service bits and stuffing indication bits in predetermined bit positions in frames are inserted into predetermined addresses in the intermediate rate frames. The intermediate rate signals are multiplexed into the high rate signal into which is inserted systematic stuffing bits and a second number of filling bits. Readout of the stores is inhibited during first intervals corresponding to the bit positions in which stuffing bits are to be inserted in the low rate component signals, second time intervals corresponding to the bit positions in which filling bits are to be inserted in the intermediate rate signals and third time intervals corresponding to the bit positions in which systematic stuffing bits and filling bits are to be inserted in the outgoing high rate signal. The system is concerned with enabling multiplexing communication between countries with different sequences of master rates to be compatible, for example, in some countries 8-34-140 Mb/s is the master rates sequence, while in others 8-140 Mb/s is the master rates. 8 Mb/s signals multiplexed as in this system can be demultiplexed either to 34 Mb/s or 8 Mb/s depending on the required receiving master rate. In the prior art, two stages of storage and stuffing have been employed with separate time bases for each channel, whereas in the present system each channel is fully set up with appropriately placed positions to receive framing, service, stuffing indication and stuffing bits, using only one time base, these channels are then stuffed as necessary and can then be multiplexed twice in succession without intermediate storage and re-timing, and further stuffing is performed systematically. Information at a nominal 8 Mb/s on input channels 1 0 -1 15  (Fig. 5) is passed via shaping and clock extraction circuits 2 0 -2 15  to the storage and stuffing circuits 3 0 -3 15 . In each of these, received information bits are stored in a shift register 30 from which they are read in response to read control timing signals from the time base 5 via a circuit 31. Any difference between the received bit rate and the timing signals is detected by phase comparator 32 and the output from this controls a stuffing authorization circuit 33 to transmit a stuffing permission signal from the time base associated with the frame to the read control circuit 31. Groups of four channels are applied to the multiplexers 4 via gates in the input leads controlled by the time base and the outputs from these are at the intermediate rate (34 Mb/s). The intermediate rate outputs are passed through further gating arrangements to a multiplexer 14 which provides an output at the 140 Mb/s rate on output 16. The time base 57 (Fig. 6) provides the timing for the 140 Mb/s multiframe and the 34 Mb/s frames, and provides the timing signals for facilitating the insertion into these frames of, for the 140 Mb/s frame, the framing, service and stuffing indication bits, and systematic stuffing bits, and for the 34 Mb/s the framing, service and stuffing indication bits. A clock 570 providing 139,264 Mb/s is applied to a series of dividers 571 (¸4), 572 (¸122) and 573 (¸6) whose outputs are applied to an insertion circuit 576 which controls gates in the outputs of circuits 575, 579 and 578 such that framing signals, stuffing indication, systematic stuffing bits and service bits can be applied at appropriate times to the intermediately multiplexed signals passed by the gates in the inputs to multiplexer 14. The insertion circuit is also controlled from a frame counter 590, since systematic stuffing is not the same in each frame of the multiframe (see below). The divider output passes via a gate 591 controlled from the frame counter to a further divider 581 (¸4), 582 (¸96) and 583 (¸4) to provide similar gate signals controlling the insertion of signals in the channel inputs to the 34 Mb/s multiplexers 4. The gate 591 is controlled by the frame counter such that the divided frequency passed to the time control terminal 5704 of the multiplexers 4 is reduced to allow for the later insertion of the filling bits in the intermediate signals. Stuffing bits for insertion in the low rate input signals are also supplied by generator 589. Timing signals from the time base established by the clock 570 and divider circuits close the input gates to the multiplexer to information signals at appropriate times and enable the stuffing and filling bits to be inserted from the generators 587-589 and 577-579 into the intermediate and high rate signals respectively. An intermediate frame (Fig. 3, not shown) incorporates 1536 bit positions in 4 sectors, the first of which provides 10 bits and 2 service bits, the remaining 3 sectors providing four bits each for stuffing indication and the last up to a further four bit positions for stuffing bits. The multiframe for the 140 Mb/s multiplexed signal incorporates stuffed frames 2, 4, 7, 9, 11, Fig. 4b (not shown) with 2928 bit positions in 6 sectors, the first providing 12 framing and 4 service bits, the remaining 5 each providing four bits for stuffing indication, while the last provides four stuffing bits. An unstuffed frame (Fig 4c, not shown) has no provision for stuffing bits.